Capacitance sensors are used to implement a variety of useful functions including touch sensors (e.g., touch pad, touch dial, touch wheel, etc.), determining the presence of an object, accelerometers, and other functions. FIG. 1 illustrates a conventional capacitance sensor 100 including three switches 105 with control terminals φ0, φ1, and φ2, an integrating capacitor 110 having a capacitance CINT, and an analog to digital (“ADC”) converter 115. Capacitance sensor 100 may be used to sense changes in capacitance on a device under test (“DUT”) capacitor 120 having a changing capacitance CDUT.
During operation, capacitance sensor 100 operates as follows to sense capacitance changes on DUT capacitor 120. First, integrating capacitor 110 is discharged to a ground potential by asserting control terminal φ0 to open circuit switch SW0 and by asserting control terminal φ1 to close circuit switch SW1. Once discharged to ground, integrating capacitor 110 is disconnected from ground by asserting φ1 to open switch SW1. Then, DUT capacitor 120 is charged to the supply voltage VS by asserting φ0 to open circuit switch SW0 and asserting φ2 to close circuit switch SW2. Once DUT capacitor 120 charges to the supply voltage VS, the charge on DUT capacitor 120 is transferred onto integrating capacitor 110 and distributed between the two capacitors. Charge transfer occurs by asserting φ1 and φ2 to open circuit switches SW1 and SW2, respectively, and asserting φ0 to close circuit switch SW0.
The above stages of charging DUT capacitor 120 and transferring the charge onto integrating capacitor 110 are repeated a fixed number times causing the voltages of nodes N1 and N2 to ramp with time as illustrated in line graphs 130 and 135, respectively. After a fixed number of consecutive charging stages and charge transferring stages, ADC converter 115 samples the final voltage on node N2. The capacitance CDUT is determined based on the output of ADC converter 115 and is proportional to the voltage at node N2 after the final charge transfer stage.
Capacitance sensor 100 functions properly if CINT is considerably larger than CDUT (e.g., 1000× larger) so as to average out noise on the supply voltage VS. However, using a large CINT value requires a large number of charge transfer cycles to achieve a good capacitance measurement of CDUT. Therefore, capacitance sensor 100 may have a relatively slow measurement time. Furthermore, capacitance sensor 100 uses ADC 115 which is a relatively complex and expensive component.